Conventionally, there is a system that a plurality of network connection apparatuses (hereinafter, referred to as “routers”) for communication with another sub-network are arranged, in parallel, with an external network thus taking a redundant arrangement within an IP (Internet Protocol) network so that, when a trouble occurs in the router operating as a master (hereinafter, referred to as a “master router”), another router in standby state (hereinafter, referred to as a “backup router”) is to act as an alternative apparatus for continuing the communication. As such a system, there is known a system using a Virtual Router Redundancy Protocol for IPv6 (see U.S. Pat. No. 5,473,599, and “Virtual Router Redundancy Protocol for IPv6”, the Internet <URL: http://www.ietf.org/internet-drafts/draft-ietf-vrrp-ipv6-s pec-03.txt>, for example).
FIG. 24 is a diagram illustrating one arrangement example of the virtual router system. In FIG. 24, there are connected to the local area network 2405, a master router 2401 for actual network connection, a backup router 2402 for the case the master router 2401 is not to be used, and host nodes 1-3 (hereinafter, referred to as “node 1”, “node 2” and “node 3”). Meanwhile, the master router 2401 and the backup router 2402 are connected respectively to exterior local area networks 2404 different from each other. In this prior art, a router group for executing VRRP is to be group-designated by means of a virtual router ID (hereinafter, referred to as “VRID”). Within the same sub-network, one is taken as a master router out of the router group having the same VRID, to actually deliver packets. As for the backup router, in the case a trouble occurs in the master router, the backup router itself switches into a master router to thereby deliver packets. The master router and the backup router are to indicate virtually the same address (virtual router IP address, virtual router MAC address) to the nodes. This allows the node to send a packet using a virtual router address as a default router, without the consciousness of whether the router for transmission is a master router or a backup router.
FIGS. 25(a) and 25(b) are sequence charts showing a switchover operation of between master and backup phases, with the use of a VRRP-defined advertisement message to be exchanged between the master and backup routers.
In particular, FIG. 25(a) is a chart showing an operation where the priority of the relevant apparatus is lower than the priority in a received advertisement message.
In FIG. 25(a), the master router 2401 sends, to the backup router, an advertisement message 2101 including a priority for itself to become a master router, by multicast at a regular interval. The backup router 2402, when receiving advertisement messages 2101 having a priority equal to or higher than its own priority from the master router within a constant time period (master down-time), resets 2102 a master down-timer and makes certain of the master 2401 being in operation.
FIG. 25(b) is a chart showing an operation where the priority possessed is higher than the priority in the received advertisement message.
In FIG. 25(b), the backup router, when receiving an advertisement message 2103 from the master router having a priority lower than the priority of its own, discards the advertisement message as it is without resetting the master down-timer (2104). The backup router, when the master down-timer expires (2105), decides that a trouble occurs in the master router and becomes itself into a master router, to send an advertisement message 2106 to the routers of the same group. Because the master down-timer has a time-out value set shorter for the higher priority and longer for the lower priority, a master router is to be established in accordance with the priority.
Incidentally, there is shown in FIG. 26 a format for an advertisement message. In FIG. 26, type field 2201 represents the type of message, wherein ‘1’ shows an advertisement message. VRID field 2202 is to describe therein a virtual-router identifier VRID representative of a virtual router group to which the relevant apparatus belongs. Priority field 2203 represents a priority to assume a master router of among the virtual router group. Advertisement interval field 2204 represents a time interval to send advertisement messages. IPv6 address field 2205 describes therein a virtual router address of the virtual router group. In the VRRP message and other fields, prescribed information is to be described in compliance with the VRRP procedure.
Meanwhile, there is disclosed, as another prior art, a technique which exchanges mutual-monitor messages at a regular interval between the master router and the backup router to thereby detect a trouble early so that, in the case of a trouble occurrence, metric value is changed to send a route information message thereby increasing the speed of switchover processing (see JP-A-7-264233).
However, with the conventional configuration described in the “Virtual Router Redundancy Protocol for IPv6”, even where the backup router receives from the master router an advertisement message whose priority is lower than its own one, it does not immediately transit to a master but discard the received advertisement message. A lapse of master downtime is waited to implement a switchover processing after a timeout. Thus, there is a problem that switchover is not effected swiftly between master and backup routers even in the presence of a router having a higher priority.
Besides switchover with a router higher in priority, router switchover process is first effected after the master router becomes unusable. For this reason, there is a problem that swift switchover is impossible between mater and backup routers in such a situation that there is a change in connectability with a network, e.g. in a mobile communication environment.
Meanwhile, the conventional configuration described in JP-A-7-264233 involves a problem that network load increases with the increase in the number of the routers taking a redundant arrangement because of the need to mutually send messages from the master router and backup router in order for state monitoring.